A lot of components are often produced by joining a plurality of workpieces, and welding is used for joints requiring high strength. Particularly spot welding, one form of resistance welding, is used to efficiently weld overlapping steel sheets such as body parts of automobiles and other vehicles (a plurality of workpieces to be joined) at a plurality of points (spots). This spot welding is generally carried out by applying a high current for a short time to workpieces to be joined through electrodes which clamp each outer side of the workpieces and thereby melting and solidifying a joint portion (a welding portion) inside the overlapped workpieces.
By the way, unlike arc welding, spot welding forms a weld internally with respect workpieces to be joined and therefore, it is difficult to directly observe the weld. Besides, in mass production processes, it is not realistic for a worker to inspect weld states of spot welding one by one due to presence of jigs, tools and so on.
Therefore, a method for non-destructively evaluating whether spot welding is good or not with high efficiency and accuracy has been looked for. Proposals for such a method are disclosed, for example, in the following Patent Documents 1 to 5. All of these documents relate to methods for evaluating spot welding based on electric resistance of a nugget portion of its vicinity of workpieces to be joined.